Antihistamines are drugs which treat allergic rhinitis and other allergies.
However, this is not always possible as some substances, such as pollen, are carried in the air, thus making allergic reactions caused by them generally unavoidable. The two largest classes of antihistamines are H 1 -antihistamines and H 2 -antihistamines. Antihistamines that target the histamine H 1 -receptor are used to "Sedating antihistamines wikipedia" allergic reactions in the nose e.
They are sometimes also used to treat motion sickness or vertigo caused by problems with the inner ear. Antihistamines that target the histamine H 2 -receptor are used to Sedating antihistamines wikipedia gastric acid conditions e. H 1 -antihistamines work by binding to histamine H 1 receptors in mast cellssmooth muscleand endothelium in the body as well as in the tuberomammillary nucleus in the brain; H 2 -antihistamines bind to histamine H 2 receptors in the upper gastrointestinal tractprimarily in the stomach.
Histamine receptors exhibit constitutive activityso antihistamines can function as either a neutral receptor antagonist or an inverse agonist at histamine receptors. Histamine produces increased vascular permeability, causing fluid Sedating antihistamines wikipedia escape from capillaries into tissueswhich leads to the classic symptoms of an allergic reaction — a runny nose and watery eyes.
Histamine also promotes angiogenesis.
Sedating antihistamines wikipedia suppress the histamine-induced wheal response swelling and Sedating antihistamines wikipedia response vasodilation by blocking the binding of histamine to its receptors or reducing histamine receptor activity on nervesvascular smooth muscleglandular cells, endotheliumand mast cells.
Itchingsneezingand inflammatory responses are suppressed by antihistamines that act on H1-receptors. H 1 -antihistamines refer to compounds that inhibit the activity of the H 1 receptor. The vast majority of marketed H 1 -antihistamines are receptor antagonists and only a minority of marketed compounds are inverse agonists at the receptor.
Sedating antihistamines wikipedia Sedation is a common side effect of H 1 -antihistamines that readily cross the blood—brain barrier ; some of these drugs, such as diphenhydramine and doxylamineare therefore used to treat insomnia.
A combination of these effects, and in some cases metabolic ones as well, lead to most first-generation antihistamines having analgesic-sparing potentiating effects on opioid analgesics and to some extent with non-opioid ones as well.
The most commonly used for the purpose include hydroxyzinepromethazine enzyme induction especially helps with codeine and similar prodrug opioidsphenyltoloxamineorphenadrineand tripelennamine ; some may also have intrinsic analgesic properties of their own, orphenadrine being an example. Second-generation antihistamines cross the blood—brain barrier to a much lower degree than the first-generation antihistamines. Their main benefit is they primarily affect peripheral histamine receptors and therefore are less sedating.
However, high doses can still induce drowsiness through acting on the central nervous system. Some second-generation antihistamines, Sedating antihistamines wikipedia cetirizinecan interact with CNS psychoactive drugs such as bupropion and benzodiazepines.
Examples of H 1 antagonists include:. The H 1 receptor inverse agonists include: H 2 -antihistamines, like H 1 -antihistamines, occur as inverse agonists and neutral antagonists. They act on H 2 histamine receptors found mainly in the parietal cells of the gastric mucosa, which are part of the endogenous signaling pathway gastric acid secretion. Normally, histamine acts on H 2 to stimulate acid secretion; drugs that inhibit H 2 signaling thus reduce the secretion of gastric acid.
H 2 -antihistamines are among first-line therapy to treat gastrointestinal conditions including peptic ulcers and gastroesophageal reflux disease. Some formulations are available over the counter.
Most side effects are due to cross-reactivity with unintended receptors. Cimetidine, for example, is notorious for antagonizing androgenic testosterone and DHT receptors at high doses.